Solution linear polymers made by free radical polymerization need to be crosslinked during or after application to improve their mechanical strength, heat resistance and chemical resistance for various high performance applications. The crosslinking reaction can be induced by chemical, thermal or actinic radiation processes. U.S. Pat. No. 5,536,759 discloses a hot melt pressure sensitive adhesive prepared from a linear, saturated polymer with pendant vinyl groups which are crosslinked by UV radiation.
High molecular weight linear polymers, which result in solution viscosities proportional to their molecular weights, are usually used in high performance applications. Attempts to lower the solution viscosities of these polymers by reducing the polymer's molecular weight leads to poor performance attributes which make them difficult to use as hot melts for high performance adhesives and coating applications. Therefore linear polymers are usually limited by their molecular weigh, and therefore also have limited uses.
To circumvent these problems, low molecular weight polymers have been made with olefinic unsaturation on side chains. The olefinic unsaturation is then crosslinked by electron beam or ultra-violet irradiation to provide sufficient cohesion and tack. Ordinarily, most of the olefinic double bonds that polymerize by free radical reactions are also thermally polymerizable. In this regard, it is known that acrylates, methacrylates, styrenes, vinyl ester, and allyl compounds, for example, are quite susceptible to thermally induced polymerization (see, Principles of Polymerization, Second Edition, George Odian, John Wiley & Sons (1981), page 214).
Star polymers having three or more polymeric arms have lower solution and melt viscosities than conventional linear polymers with the same molecular weights due to both lower radii of gyration and lower polydispersity (lower amount of high molecular weight fraction) J. Roovers in Encyclopedia of Polymer Science and Engineering, Vol. 2, Ed. 2, 1985, pp. 478-495.
Star-branched polymers, also known as radial polymers, are characterized by having three or more polymeric arms emanating from a central core. These polymers can be prepared by various polymerization procedures such as anionic, cationic, and free radical mechanisms. The star polymers are usually formed by using either multifunctional initiators, multifunctional chain transfer agents, or multifunctional coupling agents. The star polymers have unique properties including: narrow molecular weight distributions; low viscosities at low molecular weights or in solution due to their compact structures; and high viscosities at high molecular weights due to extensive entanglements.
The functionalized star polymers of the present invention, when crosslinked, can be used as a high solids solution or as hot melts for high performance adhesives and coating applications.
Radiation crosslinking (curing) of star polymers containing pendant unsaturation offers several advantages over conventional methods. The chemical reactions involved in radiation crosslinking are specific to the type of source of radiation which allows precise control of the crosslinking to achieve desired properties. Compositions designed for radiation curing polymers are not affected by the chemical or the thermal environment of the sample until they are subjected to the radiation source. The presence of the double bonds allows crosslinking at lower radiation doses compared with polymers without the double bond, for the same molecular weight. This allows long shelf life and consistent performance characteristics independent of the storage history of the product. In many cases no additional chemical agents are needed to affect the radiation crosslinking, thus leading to cleaner and simpler polymer systems. The use of star polymers that can be radiation cured offers other environmental and economic benefits due to lower emission of volatile organic compounds (VOC's) and lower energy cost due to the lower viscosities at high solids.
Although star polymer are well known, there has not been a disclosure of star polymers containing olefinic functionality for radiation curable crosslinking.
EP 0 449 413 A1 (1992) describes the use of star polymers derived from polythiols and other functional monomers which are capable of crosslinking via condensation reactions with epoxys, siloxanes or isocyanates. There is no disclosure of star branched polymers with olefinic functionality or of radiation curable compositions.
Block copolymers prepared using polythiols with a latent, or masked, thiol as a chain transfer agent have been disclosed U.S. Pat. No. 5,399,642, U.S. Pat. No. 5,492,965 and U.S. Pat. No. 5,498,675. Bulk polymerization of vinyl monomers using polythiols to prepare adhesives, coatings and molding applications have also been disclosed. U.S. Pat. No. 5,574,117 and U.S. Pat. No. 5,679,762. None of these references disclose star polymers containing olefinic functionality for radiation crosslinking.